Process for recovery of petroleum from sands and shale



Nov. 19, 1957 J. W. MARX ETAL PROCESS FOR RECOVERY OF PETROLEUM FROM SANDS AND SHALE Filed Dec. 6, 1954 2 Sheets-Sheet 1 OIL SATURATION SATURATION PERMEABILITY (M0) Alma LEGEND XI LOST CORE SANDY SHALE OR SHALEY SAND SAND DO E

INVENTORS J. W. MARX H W PARKER A TTORNEKS' Nov. 19, 1957 J. w. MARX ETAL PROCESS FOR RECOVERY OF PETROLEUM FROM SANDS AND SHALE 2 Sheets-Sheet 2 Filed Dec. 6, 1954 FIG. 3

INVENTORS J. W. MARX BY Hg; PARKER A 7' TORNEYS United States Patent Ofifice 2,813,583 Patented Nov. 19, 1957 PROCESS FOR RECOVERY OF PETROLEUM FROM SANDS AND SHALE John W. Marx, Bartlesville, Okla., and Harry W. Parker,

Evanston, Ill., assignors to Phillips Petroleum Company, a corporation of Delaware Application December 6, 1954, Serial No. 473,238

11 Claims. (Cl. 166-11) This invention relates to an improved process for recovering hydrocarbon material from strata containing same. A specific aspect of the invention relates to the production of petroleum hydrocarbons from strata of low permeability which renders usual production practices impractical.

Production of petroleum hydrocarbons from oil-bearing sands is usually effected by drilling through the oilbearing sand formation or stratum and opening up the formation around the bore hole so as to cause oil to flow into the bore hole from which it can be recovered by conventional methods. Conventional techniques of this nature depend upon the permeability of the oil sand for production from the formation. The permeability of good oil sands runs upwards from a permeability of the order of about 100 Ind. and often runs above 200 Ind. Occasionally an oil-bearing formation is discovered which is not amenable to practical commercial production or recovery by conventional methods. A large field in West Texas, with an estimated 325,000 to 500,000 commercially productive acres, comprising one of the largest reservoirs in the world, is of this category. Production from this field has been particularly disappointing, primarily because of the low bulk permeability of the matrix rock, which averages less than 1 md. This field, known as the Spraberry trend area field, has been tapped by approximately 2300 wells, and, although production has been going on in the field for approximately 6 years, less than 30 million barrels of oil have been recovered to date out of a minimum estimated 3,500 million barrels of oil in place. However, recovery of oil from the Spraberry formation has been so disappointing that further development is apparently awaiting improvement in old techniques or the development of new techniques for oil recovery.

In order to understand the problems arising from attempts to recover oil from the Spraberry formation, it is necessary to understand the peculiarities of the stratigraphy and structure of the so-called Spraberry trend. This formation comprises a laminated shale, limestone, dolomite, and sandstone section approximately 1,000 feet in thickness over most of the Midland basin. The formation comprises an upper section 300 feet in thickness and a lower section 400 feet in thickness separated by some 300 feet of currently unproductive shales. In each of these sections there is at least one sandstone stratum adjacent which are found strata comprising black shale, dolomite, and argillaceous limestone. The principal sandstone strata in each section have been variously estimated to be from 8 to 30 feet thick. The shale strata above and below the sandstone stratum in each section contain principally solid hydrocarbon material commonly referred to as kerogen, which does not flow from the formation under known production conditions. In addition to the apparently fixed nature of the solid hydrocarbon in the formation, the permeability of the sandstone, which contains fluid hydrocarbons and a lower concentration of kerogen, is unusually low, rendering it difficult to cause fluid hydrocarbons to flow from the principal sandstone stratum into a well bore penetrating the formation. Hence, production from even the principal sandstone stratum in each section of the Spraberry trend has been disappointing.

Another peculiarity of the Spraberry formation is the prevalence therein of a network of vertical fractures apparently caused by downwarping in conjunction with compaction. This intricate fracture system is directly related to the productive ability of the wells in some areas and has made limited production of oil commercially possible from the principal sandstone sections which are otherwise practically impermeable. The primary fractures are vertical and closely spaced at an average of about 10 inches, assuming a square grid pattern, so as to divide the formation into an intricate network of prismatic columns of irregular shape. These natural vertical fractures do not, in general, permit sustained commercial production from the principal sandstone layers, and it is therefore desirable to apply hydraulic fracturing which enlarges them and appreciably increases the recovery of oil therefrom. At any rate, the problem of recovering the major portion of the oil from the Spraberry trend development still exists and is a constant stimulant to improved and novel production methods which will eventually be applied to recover at least a substantial and profitable proportion of the vast reservoir of petroleum lodged in this formation.

A principal object of the invention is to provide an improved and unique process for recovering hydrocarbon material from earth strata containing the same. Another object is to provide a novel process for recovering fluid hydrocarbons from strata of low permeability containing both solid and fluid hydrocarbon materials. Another object of the invention is to provide a process for the recovery of solid hydrocarbon from a stratum containing the same. A further object is to provide an eflicient heating process for driving solid and fluid hydrocarbons from an underground formation containing the same. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

The invention comprises fracturing a formation along generally horizontal lines between bore holes in an oilbearing formation so as to provide passageways from one bore hole to the other through a stratum, recovering as much fluid hydrocarbon as possible by the usual production methods from the bore holes, and then injecting a hot fluid such as steam, superheated steam, high temperature water, or hot inert or otherwise non-deleterious gas into the formation through one or more bore holes under sufficient pressure to force the hot fluid through the artificial cracks and/or artificially enlarged natural crevices therein (effected by hydraulic fracturing). The hot fluid heats up the formation surrounding the area of injection in the bore hole and greatly decreases the viscosity of the hydrocarbon oil in the pores of the formation so as to render the same more fluid and facilitate the flow thereof from the formation to the surrounding bore holes from which the oil is recovered by conventional methods. Raising the temperature of the '5 n) formation and particularly to a temperature in the range of 400 to 750 F. resolves the solid hydrocarbon in the formation and greatly increases the effective fluid conductivity of the system, thus rendering it less difficult to cause the oil to flow through the formation to a producing Well.

In accordance with another embodiment of the invention, a chemical agent capable of facilitating the stripping of oil from the rock or shale matrix is introduced to the fluid being injected. It has been found that the stripping of oil from sands is facilitated by raising the pH of the treating fluid to above 7.5 and preferably above 8. The stripping action of hot water and/or steam is increased substantially by incorporating a water soluble alkalizing agent therein which raises the pH of the treating aqueous medium to above 7.5, such as 8 or 8.5. In vapor phase injection, ammonia is a preferred agent, while in liquid phase, water-soluble compounds of an alkaline nature are effective. Such alkaline compounds include the alkali metal compounds such as the hydroxides, carbonates, silicates, etc., as well as ammonium hydroxide. The sodium compounds are preferred. These alkaline agents may be used alone or in admixture to raise the pH of the treating fluid.

Whether steam or hot water is injected, the initial phase of the process involves a hot water flood and is thus more effective when the liquid at the flood front is alkaline. The continued addition of the alkalizing agent is not necessary after the hot water front breaks through to the producing wells, which has an economic bearing on the process. Another advantage is the fact that the alkaline additive arrests the usual corrosion process that takes place when hot steel is in contact with formation brine.

As solid and fluid hydrocarbon is recovered from the sandstone stratum and this formation is rendered more permeable to hot fluid, the area of treatment is extended to the adjacentlayer of shale and treatment of the shale to recover solid hydrocarbon is initiated. Heating and contacting the shale with hot steam converts the solid hydrocarbon or kerogen contained therein to fluid hydrocarbon and drives the same to a producing bore hole. As the solid hydrocarbon is removed by this process from the adjacent layers of shale. the some is rendered more permeable and facilitate the contacting of additional layers of. shale so as to fluidize the kerogcn so that the same can be produced from surrounding bore holes. In this manner production of solid hydrocarbon from the layers of shale adjacent the principal producing sandstone stratum can be effected.

In producing from an oil-bearing stratum adjacent a shale stratum of such low hydrocarbon content that it is not economically feasible to heat said shale stratum, the presence of a natural vertical fracture system, as in the Spraberry field, might be expected to dissipate a large quantity of heat through these fractures. However, where the adjacent layer contains a substantial proportion of argillaceous material the steam and/or hot water produces sufficient swelling of the formation to close off the vertical fractures of the escape of steam and/or hot water and thereby concentrate the available heat in a relatively thin hot zone which moves vertically through the adjacent formation. In such formation it is feasible to produce the shale formation by fracturing and hot fluid treatment in accordance with the process of the invention if economically desirable.

In a formation containing vertic'il fractures the heat from the original injection zone rapidly moves vertically through the fractures when the surrounding layers con tain no water-swellable material or an insurlicient concentration of such material to seal off the fractures in such a formation it is feasible to produce oil from a vast area above and/or below the principal sandstone formation in which production was initiated and which contained the highest concentration of liquid hydrocarbon, although it is not feasible in this instance to control the heating and producing process so as to restrict it to a relatively thin horizontal layer gradually advancing upwardly or downwardly from the main producing stratum.

According to another embodiment of the invention production from the oil-bearing formation is effected by injecting high temperature hot water, preferably at a temperature of at least 400 F., until a heated zone in the formation of a disc-like configuration has been produced with substantial lowering of the viscosity of the oil therein and conversion of kerogen to fluid hydrocarbon. When the heated zone in the formation has progressed a substantial distance horizontally or laterally through the formation, such as at least to 500 feet or more, the injection of high temperature water is discontinued and water at normal temperature, such as atmospheric temperature, is forced down the bore hole so as to displace the hot liquid from the region immediately surrounding the bore hole thereby forcing the hot zone radially outwardly from the bore hole and the point of injection. In this manner production from a relatively thin stratum can be accomplished without undue dissipation of heat into the underlying and overlying strata which may contain hydrocarbon in insufficient concentration to warrant recovery by the heating technique of the invention. The conservation of heat by this technique becomes apparent when it is realized that after oil has been produced from a disc-like zone surrounding the injection bore hole, heat is continually dissipated vertically in either direction from the hot zone as long as the zone is maintained hot. After the hydrocarbon has been produced from the initial disclike hot zone the hot zone can be advanced radially through the formation to outlying producing wells by injection of water at atmospheric temperature and driving of the hot zone in front of it. In this manner. heat from the zone already produced is moved outwardly from the injection well with the heat from the formation going into the cooler water and movement of the absorbed heat through the formation intermediate the hotter and cooler injected water. This manner of operation is more conserving of heat than is continued injection of hot fluid in that it maintains a hot zone in the formation only where fluid hydrocarbon is being produced, and is particularly applicable to recovery of oil from an oilbearing stratum which is adjacent layers in the formation of extremely low or zero oil concentration.

According to another embodiment of the invention, after steam injection has been continued for a suiTicient length of time and with sufficient volumes of steam to heat up a desired zone in the formation. the injection of steam is discontinued and the pressure on the fluid in the bore hole and surrounding oil-bearing stratum is reduced so as to permit the condensed water or hot connate water of the formation to be converted to steam thereby distilling and driving additional fluid hydrocarbons from the formation. During the injection of super-heated steam the pressure in the bore hole and in the formation is raised to a pressure of the order of 500 to 5,900 pounds per square inch and the temperature of the formation in contact with and closely adjacent the hot hold is raised to considerably above the normal boiling point of water and when this pressure is reduced, the water in the pores of the formation is rapidly vaporized and converted to steam so as to drive fluid hydrocarbons out of the formation into the producing wells as well as into the injection well.

Tests were performed upon oil sand and slide cores from a well in the Spraberry trend area field. Oil concentration, connate water, porosity, and permeability were determined by conventional methods and the yield calculations based upon saturation data are believed to be as representative of the formation as modern core sampling technique-will permit. The results obtained are presented in the following table.

Legend Q Oil produced by solution gas drive from small fragments or frozen core under laboratory conditions. pressure depleted to one atmosphere.

Qsn2=0il produced by steam distillation at 212 F. and one atmosphere pressure. Qsoo=0i produced by steam dlstlllation at 400' E. and one atmosphere pressure.

Actual reservoir injection pressures in excess of 1000 p. s. 1., with saturation temperatures greater than 500 F., would be required in the field itself.

By careful calculation it has been found that it is economically feasible to recover oil from the formation through either the upper or lower oil sand by steam or hot water injection in accordance with the invention. In addition, the laboratory test data qualitatively indicate an increase in the rate of recovery. Oil production by steam distillation appeared to be complete in about one hour while solution gas drive production from the rock fragments of the same size required more than 24 hours for completion at room temperature. This high rate of producing from an oil sand renders it highly advantageous to heat up an appreciable disc-like area surrounding the injection point and then driving the hot zone outwardly to produce additional area. By injecting hot steam for a predetermined calculated time sufficient to establish a large hot zone, and following this injection with water injection at ambient temperature, the production from any given area is limited to the approximate time of hot fluid injection and the dissipation of heat vertically from the formation is likewise limited to this period of time.

The data in the table clearly illustrate the advantage of heating the aqueous injection fluid to a temperature in the range of 400 to 750 F. This high temperature in the formation not only increases production from the oil sands but also converts kerogen in the shale formation to fluid hydrocarbon which is readily recoverable, whereas temperatures of a lower order are not at all eflectivc in converting kcrogcn to fluid hydrocarbon material. Heating with a non-aqueous fluid requires tempcraturcs of 800 to 1000 F. to obtain similar conversion of kcrogcn to that with an aqueous medium at 400 to 750 F.

Thus it appears that hot fluid injection and particularly steam and hot water injection offer an excellent means of increasing both the cumulative recovery and the rate of recovery in a formation such as the Sprabcrry forma tion which has particularly low permeability. Formations of a low order of permeability, such as below about 25 md., are particularly attractive for application of the process of the invention in its various aspects. It must be noted that where the permeability of a formation is of this order it is absolutely essential to fracture the formation so as to provide horizontal fractures therein which make it possible to inject the hot fluid into the formation with the necessary rapidity for practical production from the formation. Without progress of the hot zone through the formation at a reasonably rapid rate, dissipation of heat into the subjaccnt and superimposed layers may well be a deterrent to commercial success of the technique of the invention.

It is worth noting that steam injection yields a rapid initial rate, not from a distillation process, but because of other production mechanisms which operate concurrently. Utilizing this technique, evidence may be visualized as follows:

(1) Initially, condensed water floods standing oil out of fractures.

(2) Hot water imbibition produces some oil from the preferentially water-wet sands.

(3) Raising the temperature in the fracture system reduces hydrocarbon viscosity generally, melting some hydrocarbons, such as paraffins, thus opening plugged channels and expediting normal flow processes.

(4) Liquid vapor pressures are increased as the temperature rises thereby increasing the available solution gas driving energy.

(5) Actual steam distillation removes hydrocarbon by vapor transport through the heated artificial crevices.

(6) At temperatures above 400 F. appreciable rcsolution of the kerogcn or solid shalc hydrocarbon is effected.

Hydraulic fracturing to produce artificial horizontal cracks in the formation is effected by any suitable means, such as the conventional methods of the art, wherein a heavy fluid such as jcllicd gasoline (napalm) is injected into the formation around the bore hole with sufficicnt pressure to fracture the formation and thereafter the jellied gasoline is rendered more fluid by suitable solvents and recovered from the formation. Hydraulic fracturing is also effected with suitable heavy liquids such as heavy crude and/or refined oil, utilizing a suitable propping agent, such as suspended coarse sand, in the oil to hold the cracks open after release of pressure. Any hard, solid, particulate material may be used as a propping agent. Recently extremely rapid injection rates have been used to inject a large volume of heavy oil in a short period of time with outstanding success. Sand is used in the last batch of oil injected to hold open the cracks after release of pressure. However, the specific method of hydraulic fracturing utilized is not a part of the invention.

A more complete understanding of the invention may be had by reference to the drawing of which Figure 1 is a chart showing the oil saturation, water saturation, porosity, and permeability of the upper and lower oil sands in the Sprabcrry trend field in a representative well; F'gurc 2 is an illustrative pictorial view of a section of thc Sprabcrry formation showing the natural vertical fracture network and the artificial horizontal fractures formed in the practice of the invention; and Figure 3 is a schematic elcvational view, partly in section, of a pair of spaced-apart wells adapted for injection and/or production.

Referring to Figure l, the principal oil producing sand in the upper section of the formation is at 6550 feet and at 7440 feet in the lower section, as indicated by the arrows. This oil-producing stratum in the upper section has a depth or thickness of approximately 8 feet while the principal producing sand in the lower section has a depth of about 10 or 11 feet. The water saturation of these sands is relatively low and the porosity and permeability are relatively high as compared with the remainder of the formation but are particularly low when compared to the porosity and permeability of the usual better oilproducing sands. It can be seen from the chart that the principal producing formations are substantially perccnt oil sand While the other strata in the formation contain substantial amounts of shale, dolomite, and limestone.

Referring to Figure 2, an illustrative, generally prismatic section of the Sprabcrry formation is represented by numeral 10. This section is taken so as to show the vertical fracture system through one of the principal sandstone strata 12 and a portion of the adjacent shale strata 14 and 16. A natural network of vertical fractures 18 traverses the formation caused by downwarping in conjunction with compaction of the formation. Fractures represent artificial crevices or cracks induced in the formation horizontally by hydraulic fracturing. These artificial cracks or crevices open up the formation to the passage of high temperature fluid from one well or bore hole to another in the formation. The Wells are positioned at spacedapart locations which permit operation in accordance with the invention. It has been found that wells spaced from A; to /2 mile apart can be utilized in the manner described to produce fluid hydrocarbons by passage of steam or other high temperature fluid from an injection well to surrounding producing wells. As an illustration, the hydraulic fluid utilized in hydraulic frac luring was produced in one of the production wells approximately /11 mile distance from the injection well with in 24 hours from the fracturing operation.

Referring to Figure 3, a conventional well 21 having a casing 22 and a tubing 24 in a bore hole 25 penetrates an oil sand 26 and adjacent shale strata 28. The tubing is connected by a valved line 30 with a superheater 32 and this superheater is connected by a line 34 to a boiler 36 of any suitable type. A line 38 leading into boiler 36 provides water or other suitable fluid for heating. A suitable high pressure pumping means 37 in line 30 is provided to force fluid into the Well tubing under suitable pressure. In applications where a gas other than steam is to be used as the hot fluid, superheater 32 and boiler 36 may be replaced by any conventional heater suitable for heating the particular gas being utilized. In instances where hot flue gas is the heating fluid, the same may be produced by combustion in any type of furnace and this may be admixed with steam or injected directly from the furnace into the well bore. A valved line 39 is provided for introduction of a suitable chemical agent, such as a water-soluble alkaline material, to the injecting line 30. Such agent may also be added at other suitable points.

A line 49 connects the Well head with a condenser 42 having inlet and outlet lines 43 and 44, respectively, for passage of cold fluid through the condenser. Line 45 connects the lower section of condenser 42 with water storage tanlt 46. A line 47 connects the condenser with a recovery system 48 adapted to separate the hydrocarbons into a gaseous phase and a liquid phase. The gases are recovered overhead through line 50 and the liquid phase is recovered through line 52. In some instances gas liquid separation is effected in the well by conventional production methods.

Water tank -36 is connected by means of line 54 with tubing 24 and a pump 56 in line 54 is utilized to force water under pressure into tubing 24 when moving a hot zone outwardly from the injection well with cool water injection. The condensed water from tank 46 may also be used as a portion of the hot water or steam source for injection.

A line 59 connecting with the well head is utilized to apply suction pressure or vacuum to the fluid in the well and formation when desired. A packer 60 in the annulus 62 around tubing 24 is utilized to pack off the annulus during the hydraulic fracturing step and is re movable thereafter so as not to interfere with production.

A second well. B, represents a producing well spaced apart from injection well A a suitable distance to permit production in accordance with the invention. Any number of these producing wells 8 are located around well A so as to receive the fluid hydrocarbons forced out of the formation by injection of hot fluid through well A into the oil sand 26. Lines 63 and 64 carry nd oilcwntaining fluids from well A. It is usuri desirable to position 4 or 5 wells 8 in a generally ar pattern around well A. although a smaller or larger number may be utilized where conditions warrant. Neil B has the same or similar elements to well A but the injection means shown in connection with Well A are not shown on well B. However, as the oil field in which the wells are drilled is developed, it may be convenient to utilize any ill) one of wells B as an injection well in which case the injection apparatus shown in connection with well A will be utilized on well B.

In operation utilizing water and steam as the injection fluid, water is fed to boiler 36 through line 38 where it is converted to steam and the steam is passed via line 34 into superheater 32. Superheater 36 is heated or fired by any conventional means and the temperature of the steam is raised to the desired level and the superheated steam is then injected via line 30 into tubing 24 from which it travels to the oil sand 26. It is assumed that oil sand 26 has been fractured horizontally so as to provide passageways laterally through the sand stratum radially from the bore hole. The hot steam moves outwardly and radially from bore hole 25 and progresses through the oil sand to form a generally disc-shaped hot producing zone. The injection of hot fluid may be continued until the producing zone approaches producing wells B, or operation may be conducted as described above with outward movement of the producing zone by injection of water following the initial steam injection.

In utilizing the embodiment of the invention wherein reduced pressure in the producing formation is established so as to cause connate and condensed water to boil and form steam in the pores of the formation the injection of hot fluid is discontinued and suction is applied to the injection well and/or to the producing wells through a line connecting with the wellhead, such as line 59 by means of any suitable suction device so as to reduce the pressure in the oil sand, thereby reducing the boiling point of the trapped water sufficiently to cause distillation and additional production of oil from the sand. In this type of operation repeated steam injection or hot water injection followed by pressure reduction to cause distillation may be utilized.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process for recovering hydrocarbon from a hydrocarbon-containing stratum penetrated by at least two spaced-apart boreholes comprising hydraulically fracturing an area in said stratum surrounding said boreholes so as to form artificial cracks or crevices in said stratum connecting said boreholes; introducing a propping agent into said cracks or crevices to hold same open; thereafter injecting hot fluid into said formation through one of said boreholes so as to heat up the hydI'OCZllbOTlCOnlfll[ling i stratum to a temperature of at least 400 F. and drive hydrocarbon from the surrounding stratum to at least one other of said boreholes; and recovering from at least one other of said boreholes hydrocarbon driven from said stratum by said but fluid.

2. The process of claim 1 wherein said hot fluid comprises steam.

3. The process of claim 2 wherein ammonia is incorporated in said steam during the initial stages of the injection in an amount sufficient to assist in the stripping of oil from the stratum and in reducing corrosion in the well piping.

4. The process of claim 1 wherein said hot fluid is aqueous and a chemical rent capable of assisting in the stripping of oil from the ;:tun1 is incorporated in the aqueous fluid at least in the initial phase.

5. The process of claim 4 wherein a water-soluble alkaline material is adiled to the aqueous heating fluid during the initial stinger injec ion in sufficient amount to raise the pH of the i. cd fluid to at least 8.

6. A process for recovering lluid hydrocarbon from a stratum containing same and also a substantial proportion of solid hydrocarbon material convertible to fluid hydrocarbons at temperatures in the range of 400 to 750 F., said stratum having a low permeability which renders impractical the production of oil therefrom by forcing a high temperature fluid between reasonably spaced boreholes, which comprises first fracturing said stratum horizontally between a pair of boreholes so as to provide a path for fluid flow from one to the other of said boreholes; introducing a propping agent into said path to maintain same open when fracturing pressure is released; injecting a hot aqueous fluid at a temperature of at least 400 F. into one of said boreholes so as to heat said stratum to a temperature in said range suflicient to convert at least a portion of said hydrocarbon to fluid hydrocarbons and greatly increase the fluidity of hydrocarbons therein; continuing the injection of fluid so as to drive fluid hydrocarbons to the other borehole; and recovering said fluid hydrocarbons from said other borehole.

7. The process of claim 6 including injection of hot Water so as to heat a disc-like zone in said stratum surrounding said borehole and then injecting water of normal temperature into the borehole so as to drive the resulting disc-like zone laterally along the stratum and produce additional hydrocarbon in at least one surrounding borehole.

8. The process of claim 6 wherein said hot fluid comprises essentially steam.

9. T he process of claim 8 including the steps of intermittently reducing the fluid pressure in said stratum while the stratum is hot so that water in the pores in the stratum is converted to steam thereby distilling and driving additional hydrocarbon fromsaid stratum and recovering same from at least one of said boreholes.

10. The process of claim 9 wherein the fluid pressure adjacent the borehole is reduced to less than atmospheric.

11. A process for recovering fluid and solid hydrocarbon material convertible to fluid hydrocarbons at temperatures in the range of 400 to 750 F. from a formation containing same, said formation comprising a permeable stratum containing principally fluid hydrocarbon material and at least one adjacent substantially less permeable stratum containing principally solid hydrocarbon material, which comprises extracting fluid hydrocarbon through at least two boreholes penetrating said formation; hydraulically fracturing said permeable stratum from said boreholes so as to form horizontal cracks or crevices providing fluid flow between said boreholes; introducing coarse sand to said cracks or crevices to hold same open when fracturing pressure is released; thereafter injecting a hot fluid comprising essentially steam into one of said boreholes so as to heat said permeable stratum to a temperature in the range of 400 to 750 F. and distill therefrom additional fluid hydrocarbon; continuing the injection of hot fluid into said permeable stratum so as to heat at least a substantial portion of the adjacent less permeable stratum and convert solid hydrocarbon therein to fluid hydrocarbon and drive same to the other borehole; and recovering fluid hydrocarbon from said other borehole.

References Cited in the file of this patent UNITED STATES PATENTS 1,237,139 Yeomans Aug. 14, 1917 1,491,138 Hixon Apr. 22, 1924 1,651,311 Atkinson Nov. 29, 1927 2,068,979 Fisher Jan. 26, 1937 2,246,726 Garrison June 24, 1941 2,357,559 Smith Sept. 5, 1944 2,596,843 Farris May 13, 1952 2,630,182 Klotz, Jr. Mar. 3, 1953 2,642,943 Smith et al. June 23, 1953 2,687,179 Dismukes Aug. 24, 1954 FOREIGN PATENTS 156,396 Great Britain Jan. 13, 1921 OTHER REFERENCES Vertical Hydraulic Fracturing, R. C. Clark, Jr., et al., appearing in: What the Journal has published on Hydraulic Fracturing; copyright 1954, The Oil and Gas Journal and the Petroleum Publishing Co., Tulsa, Oklahoma, page 10 relied on. 

1. A PROCESS FOR RECOVERING HYDROCARBON FROM A HYDROCARBON-CONTAINING STRATUM PENETRATED BY AT LEAST TWO SPACED-APART BOREHOLES COMPRISING HYDRAULICALLY FRACTURING AN AREA IN SAID STRATUM SURROUNDING SAID BOREHOLES SO AS TO FORM ARTIFICAL CRACKS OR CREVICES IN SAID STRATUM CONNECTING SAID BOREHOLES; INTRODUCING A PROPPING AGENT INTO SAID CRACK OR CREVICES TO HOLD SAME OPEN; THEREAFTER INJECTING HOT FLUID INTO SAID FORMATION THROUGH ONE OF SAID BOREHOLES SO AS TO HEAT UP THE HYDROCARBON-CONTAINING STRATUM TO A TEMPERATURE OF AT LEAST 400*F. AND DRIVE 